Messages in this thread | | | Date | Mon, 27 Jan 2020 11:07:52 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH] KVM: arm64: Treat emulated TVAL TimerValue as a signed 32-bit integer |
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Hi Alexandru,
On 2020-01-27 10:36, Alexandru Elisei wrote: > According to the ARM ARM, registers CNT{P,V}_TVAL_EL0 have bits [63:32] > RES0 [1]. When reading the register, the value is truncated to the > least > significant 32 bits [2], and on writes, TimerValue is treated as a > signed > 32-bit integer [1, 2]. > > When the guest behaves correctly and writes 32-bit values, treating > TVAL > as an unsigned 64 bit register works as expected. However, things start > to break down when the guest writes larger values, because > (u64)0x1_ffff_ffff = 8589934591. but (s32)0x1_ffff_ffff = -1, and the > former will cause the timer interrupt to be asserted in the future, but > the latter will cause it to be asserted now. Let's treat TVAL as a > signed 32-bit register on writes, to match the behaviour described in > the architecture, and the behaviour experimentally exhibited by the > virtual timer on a non-vhe host. > > [1] Arm DDI 0487E.a, section D13.8.18 > [2] Arm DDI 0487E.a, section D11.2.4 > > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Huhuh... Nice catch!
Fixes: 8fa761624871 ("KVM: arm/arm64: arch_timer: Fix CNTP_TVAL calculation")
(how many times are we doing to fix this???)
> --- > include/kvm/arm_arch_timer.h | 2 ++ > virt/kvm/arm/arch_timer.c | 3 ++- > 2 files changed, 4 insertions(+), 1 deletion(-) > > diff --git a/include/kvm/arm_arch_timer.h > b/include/kvm/arm_arch_timer.h > index d120e6c323e7..be912176b7a3 100644 > --- a/include/kvm/arm_arch_timer.h > +++ b/include/kvm/arm_arch_timer.h > @@ -10,6 +10,8 @@ > #include <linux/clocksource.h> > #include <linux/hrtimer.h> > > +#define ARCH_TIMER_TVAL_MASK ((1ULL << 32) - 1) > + > enum kvm_arch_timers { > TIMER_PTIMER, > TIMER_VTIMER, > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c > index f182b2380345..5d40f17f7024 100644 > --- a/virt/kvm/arm/arch_timer.c > +++ b/virt/kvm/arm/arch_timer.c > @@ -805,6 +805,7 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu > *vcpu, > switch (treg) { > case TIMER_REG_TVAL: > val = timer->cnt_cval - kvm_phys_timer_read() + timer->cntvoff; > + val &= ARCH_TIMER_TVAL_MASK;
nit: Do we really need this mask? I'd rather see it written as
val = lower_32_bits(val);
> break; > > case TIMER_REG_CTL: > @@ -850,7 +851,7 @@ static void kvm_arm_timer_write(struct kvm_vcpu > *vcpu, > { > switch (treg) { > case TIMER_REG_TVAL: > - timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + val; > + timer->cnt_cval = kvm_phys_timer_read() - timer->cntvoff + (s32)val; > break; > > case TIMER_REG_CTL:
Otherwise, looks good to me. If you're OK with the above change, I'll take it as a fix.
Thanks,
M. -- Jazz is not dead. It just smells funny...
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